A so-called GTL process for producing liquid hydrocarbons that contain fuel oil from natural gas is described, for example, in WO 2007/114274 A1. FIG. 2 of the accompanying drawings is a schematic illustration of the flow of such a known GTL process.
The GTL process illustrated in FIG. 2 includes a hydrodesulfurization step 120 of hydrodesulfurizing sulfur compounds in natural gas, a synthesis gas production step 130 of producing synthesis gas by way of a reforming reaction of natural gas and steam and/or carbon dioxide, a carbon dioxide removal step 140, which is provided whenever necessary, a Fischer-Tropsch oil production step 150 of producing Fischer-Tropsch (FT) oil from the synthesis gas by way of Fischer-Tropsch (FT) synthesis, an upgrading reaction step 160 of hydrogenating the produced Fischer-Tropsch oil and an upgrading gas/liquid separation step 170 of subjecting the hydrogenated product obtained by the upgrading reaction step to gas/liquid separation to obtain liquid hydrocarbons, the above steps being arranged sequentially from the natural gas feed side (or the upstream side) or from the left side in FIG. 2.
The synthesis gas produced from the synthesis gas production step 130 partly branches off at a stage prior to getting to the Fischer-Tropsch oil production step 150 to form a branch line 145 as shown in FIG. 2 and the synthesis gas in the branch line 145 is separated into high-purity hydrogen (line 192) and off gas (line 191) in a hydrogen separation step 190 typically by means of a hydrogen PSA (pressure swing adsorption) method. The separated high-purity hydrogen joins hydrogen circulation line 177, by way of lines 192 and 197, where hydrogen circulates from the upgrading gas/liquid separation step 170 to the upgrading reaction step 160. On the other hand, the off gas that is purged from the line 191 is normally consumed as fuel.
Since the off gas 191 discharged from the hydrogen separation step 190 contains unreacted methane, a significant improvement can be achieved in terms of raw material consumption per product to an economic advantage if it can be taken into the process once again and reutilized as raw material. However, there is not any technique proposed to date to realize a process for treating such off gas.